For more up-to-date nvidia drivers, you need to add another PPA. As of 12.04, this is still necessary for Nvidia GT 6xxM cards. It may be optional for the GT 4xxM and GT 5xxM series on 12.04. When in doubt, just install it. The command is:

sudo add-apt-repository ppa:ubuntu-x-swat/x-updates

sudo apt-get update

To install Bumblebee using the proprietary nvidia driver:

sudo apt-get install bumblebee bumblebee-nvidia linux-headers-generic

if you are only interested in power savings, don't do above step. Instead execute